This invention relates to releasable fastening apparatus and, more particularly, to a releasable fastening apparatus utilizing a shape memory alloy actuator.
Releasable fastening devices are commonly used in aerospace vehicles and in other applications wherein which it is necessary to interconnect mutually adjacent structures which must subsequently be quickly and reliably disconnected. One application of such fastening devices is the interconnection of spacecraft components which must be separated during launch or in a subsequent phase of a mission. For example, the external fuel tanks of certain space vehicles are rigidly connected to the vehicles by means of pyrotechnically actuated fasteners, termed explosive bolts, for permitting release and jettisoning of the tanks after the fuel has been exhausted. Such fasteners are actuated by pyrotechnic squibs, by means of electrical current conducted through resistive elements adjacent the respective explosive charges, whereby a plurality of fasteners may be released by the application of appropriate levels of current.
Whereas such pyrotechnic fasteners have proved useful in certain applications, they entail several limitations and disadvantages. A major disadvantage is that they entail hazards in storage and handling, resulting in increased costs for maintaining safety precautions during transportation, storage, and use. Additionally, explosive release mechanisms are susceptible to inadvertent detonation from brief pulses of electrical charges, and they must be carefully isolated from any potential sources of such stray electrical pulses.
A further disadvantage of such pyrotechnic fastener mechanisms is that their detonation adjacent spacecraft components imparts physical shock and vibration which may be transmitted to delicate components such as integrated circuits, circuit boards, and the like. The detonation of such explosive charges may thus result in damage to sensitive components. Additionally, shock and vibration are transmitted to the crew. A further disadvantage is that the pyrotechnic release mechanisms cannot be nondestructively tested, because the squibs are consumed during use. Although such release mechanisms are generally reliable, it is important in certain applications that the fastener mechanisms be fully tested prior to use. Another disadvantage of such pyrotechnic fastener mechanisms is that they are not capable of repeated use. Thus, they are not suited for applications in which two structures must be connected and disconnected repeatedly, without substantial down time for refurbishing the release mechanisms.
Other prior-art fastener devices have entailed electromechanically actuated release mechanisms. Such devices generally are not sufficiently reliable for use in critical aerospace applications and often entail excessive weight and complexity.